Solubility-product constant and thermodynamic properties for synthetic otavite, CdCO 3(s) , and aqueous association constants for the Cd(II)-CO 2 -H 2 O system

Details Solubility-product constant and thermodynamic properties for synthetic otavite, CdCO 3(s) , and aqueous association constants for the Cd(II)-CO 2 -H 2 O system Solubility-product constant and thermodynamic properties for synthetic otavite, CdCO 3(s) , and aqueous association constants for the Cd(II)-CO 2 -H 2 O system

Jun 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993gecoa..57.2699s&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 57, Issue 12, pp.2699-2713

Physics

6

Scientific paper

Considerable disparity exists in the published thermodynamic data for selected species in the Cd(II)-CO 2 -H 2 O system near 25°C and 1 atm pressure. Evaluation of published experimental and estimated data for aqueous cadmium-carbonate species suggests an association constant, pK , of -3.0 ± 0.4 for CdCO 3 0 , about -1.5 for CdHCO 3 + , and -6.4 ± 0.1 for Cd(CO 3 ) 2 2- ( T = 298.15 K ; P = 1 atm; I = 0). Examination of all available data for cadmium-hydrolysis species and - Cd ( OH ) 2( s ) ) confirms that the consistent set of constants presented by and ( Hydrolysis of Cations , 1976) is the best available. The solubility of synthetic otavite, CdCO 3(s) , has been measured in KClO 4 solutions where I 0.1 M . We calculated pK sp = 12.1 ± 0.1 ( T = 25.0° C ; P = 1 atm; I = 0) from measured concentrations of Cd 2+ , measured P C 0 2 and pH, our selected set of equilibrium constants, and activity corrections estimated using the Davies equation. Values at 5 and 50°C were 12.4 ± 0.1 and 12.2 ± 0.1, respectively. Based on the new solubility data and the CODATA key values for Cd 2+ and CO 3 2- , a new set of thermodynamic properties is recommended for otavite: G f 0 = -674.7 ± 0.6 kJ / mol ; H f 0 = -751.9 ± 10 kJ / mol ; S 0 = 106 ± 30 J/mol K; and G r 0 for the reaction Cd 2+ + CO 3 2- CdCO 3 ( s ) is -69.08 ± 0.57 kJ / m

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